Blood pump

ABSTRACT

The invention concerns a control device for controlling a blood flow of an intravascular blood pump for percutaneous insertion into a patient&#39;s blood vessel, the blood pump comprising a pump unit with a drive unit for driving the pump unit and configured to convey blood from a blood flow inlet towards a blood flow outlet, wherein the control device is configured to operate the blood pump in a selectable zero-flow control mode, wherein a blood flow command signal is selected, and the control device comprises a first controller and a second controller, wherein the first controller is configured to control the blood flow by adjusting a speed command signal for the drive unit, and the second controller is configured to control a drive speed of the drive unit.

FIELD OF THE INVENTION

The present invention concerns a blood pump, particularly anintravascular blood pump for percutaneous insertion into a patient'sblood vessel. In particular, the invention relates to a specific controlmethod of a percutaneous insertable blood pump, and a correspondingcontrol device as well as a system comprising the control device and theblood pump. While the present invention is configured for andparticularly useful for intravascular blood pumps, it is less relevantfor larger blood pumps, such as VADs that are not placed inside a bloodvessel or inside the heart, but outside the patient's heart, e.g.implanted in the thoracic cavity.

INTRODUCTION

Ventricular Assist Devices (VADs) are used to support the function of apatient's heart, either as a left ventricular assist device (LVAD) orright ventricular assist device (RVAD). While typical VADs are connectedto the patient's heart by means of suitable conduits and are implantedinto the patient's thoracic cavity outside the heart, an intravascularblood pump for percutaneous insertion typically comprises a catheter anda pump unit and is inserted through an access into a blood vessel andfurther into the patient's heart, e.g. through the aorta into the leftventricle. The pump unit may be located at the distal end of thecatheter and comprises a blood flow inlet and a blood flow outlet and acannula through which the blood flow is created e.g. by a rotor orimpeller of the pump unit. For example, the cannula may extend throughthe aortic valve with the blood flow inlet disposed at a distal end ofthe cannula in the left ventricle and the blood flow outlet disposed ata proximal end of the cannula in the aorta. By creating the blood flow,the pressure difference between the outlet and the inlet is overcome.

An important aspect of intravascular blood pumps (hereinafter alsosimply referred to as “blood pumps”), amongst others, is explanting theintravascular blood pump from a patient and therefore affirming that thenatural heart function has recovered. This may be done, for example, byadequately reducing the amount of assistance provided by the blood pump,so that the blood pump can be finally explanted once the heart is foundsufficiently recovered. This aspect, i.e. determining the exact point intime for explantation, is not that important in larger VADs, which aretypically implanted in a patient's thoracic cavity and designed forlong-term applications.

Up to now, there is no physical signal known that sufficientlyrepresents the status of heart recovery as long as an intravascularblood pump is implanted. While the blood pump is assisting the heart, itis not possible to know the unassisted heart function. And when theblood pump is switched off flow regurgitation through the cannula occursso that it is impossible to know about the unassisted heart function.Regurgitation is particularly a problem of intravascular blood pumpsbecause the blood pump, more specifically the pump's cannula, extendsthrough a cardiac valve, e.g. the aortic valve, thereby creating an openpath through the cardiac valve, which backflow into the heart when theblood pump is not driven. This issue typically does not occur inextravascular devices because they do not extend through a cardiac valvebut by-pass the cardiac valve, such as VADs which are arranged outsidethe heart, e.g. in the thoracic cavity.

In the state-of-the-art the current pump speed setting is manuallyreduced by a physician gradually, e.g. by one level, based onprofessional experience. After reduction of the pump speed, the averageaortic pressure is monitored. Some institutions perform echography basedleft ventricular volume assessments and continuous cardiac outputmeasurements. If the mean aortic pressure remains stable it is assumedthat the heart is able to take over work from the blood pump. However,if the mean aortic pressure drops, it is assumed that the heart stillneeds more assistance so that the pump speed needs to be increasedagain. Further, before blood pump explantation, a so-called on/offapproach is applied. In doing so, the pump speed is significantlyreduced, e.g. for some hours, in which the patient's physiologicalcondition and the ventricular expansion in particular are observed, e.g.based on echocardiography (ECHO) measurements and/or cardiacventriculography. ECHO may provide information on the heart such as sizeand shape, e.g. internal chamber size quantification, pumping capacity,and allow a calculation of the cardiac output, ejection fraction, anddiastolic function. Cardiac ventriculography involves injecting contrastmedia into the heart's ventricle to measure the volume of blood pumped.Measurements obtained by cardiac ventriculography are ejection fraction,stroke volume, and cardiac output.

When the blood pump is switched off and if the heart still worksinsufficiently, the no longer assisted ventricle would significantlydilate so that not enough blood volume is ejected from the ventricleduring the systolic phase, leading to an increase in left ventricularend-diastolic volumes and pressures. That is to say, due to thereduction of the pump speed the heart is loaded what may correspond toan acute overload of a still not recovered heart. This may result in asetback of the therapy of several days.

Thus, the actual monitoring process before explantation of a blood pumpis more or less a trial-and-error procedure, in which, if the patient'scondition remains stable, the pump speed is further reduced, and if itbecomes worse, the pump speed needs to be increased again.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved controlmethod and correspondingly improved control device for an intravascularblood pump as well as a system comprising the control device and anintravascular blood pump, wherein the blood pump can be operated suchthat a better assessment of the status of heart recovery can beascertained.

The object is achieved by the features of the respective independentclaims. Advantageous embodiments and further developments are defined inthe respective dependent claims.

For sake of clarity, the following definitions will apply herein:

The term “characteristic parameter of the heart” is to be understood asa particular value derived from a physiological signal that is able tocharacterize a heart's condition with respect to, for example, loading,such as overloaded or unloaded, and/or a physiological condition, suchas weak, strong, or recovered.

The “human circulatory system” is an organ system that permits blood tocirculate. The essential components of the human circulatory system arethe heart, blood and blood vessels. The circulatory system includes thepulmonary circulation, a “loop” through the lungs where blood isoxygenated; and the systemic circulation, a “loop” through the rest ofthe body to provide oxygenated blood.

The herein disclosed improvement concerns blood pumps that comprisesettable blood flow levels. For example, in case of a rotational bloodpump, “settable blood flow levels” may be discrete blood flow levels orcontinuously settable blood flow levels in a range defined by a minimumblood flow and a maximum blood flow.

The basic idea of the herein proposed control device and correspondingcontrol method for controlling an intravascular blood pump is to providea mode in which the current blood flow through the blood pump can bekept very low compared to its blood flow outlet capacity, preferably atzero. Preferably, the blood flow through the blood pump is kept between0 and 1 L/min, more preferable between 0 and 0.5 L/min, between 0 and0.2 L/min or even between 0 and 0.1 L/min. Most preferably, the bloodflow through the blood pump is kept at about zero flow. In that case theblood pump is controlled such that it does neither produce a positivenor a negative blood flow. This operation mode is herein called“zero-flow control mode”. For example, a zero-flow may be establishedand/or maintained by controlling a drive unit, e.g. a motor, of theblood pump, in particular the current drive speed of the drive unit,e.g. motor, and thereby the blood flow such that just the currentpressure difference between the blood flow inlet and the blood flowoutlet of the blood pump is compensated.

“Zero-flow” within this context has to be understood as zero flow or avery low blood flow. As the purpose of the zero-flow control mode is toachieve knowledge about a recovery status of the heart, it may also beadequate to perform a very low blood flow. A low blood flow, such as upto 0.1 L/min, up to 0.2 L/min or even up to 0.5 L/min, shall be regardedas “zero-flow” within this context. In any case “zero-flow” shall not benegative. In other words, the zero-flow shall not allow any back flowthrough the blood pump.

An intravascular blood pump for percutaneous insertion within thiscontext comprises a catheter and a pump unit and is inserted into thepatient's heart via a blood vessel, e.g. through the aorta into the leftventricle. The pump unit comprises a blood flow inlet and a blood flowoutlet and a cannula through which the blood flow is created by a driveunit for driving the pump unit. For example, the pump unit may comprisea rotor or impeller that is driven by a drive unit, e.g. a motor, toconvey blood from the blood flow inlet towards the blood flow outlet.For example, the cannula may extend through the aortic valve with theblood flow inlet disposed at a distal end of the cannula in the leftventricle and the blood flow outlet disposed at a proximal end of thecannula in the aorta. The intravascular blood pump may have a maximumouter diameter in the range of about 12 French (F) (about 4 mm) to about21 French (F) (about 7 mm), e.g. 12 F (about 4 mm), 18 F (about 6 mm) or21 F (about 7 mm), which typically is the maximum outer diameter of thepump unit. The catheter may have an outer diameter, which is less thanthat of the pump unit, e.g. 9 F (about 3 mm).

The natural heart function creates a pressure difference, for examplebetween the aorta and the left ventricle. In order to create a positiveblood flow, the blood pump has to overcome this pressure difference.Otherwise, i.e. if the pressure created by the blood pump is too low,the existing pressure difference between the aorta and the leftventricle will cause a backflow into the left ventricle.

By applying the zero-flow control mode, the blood pump does not supplyassistance or very low assistance to the heart and advantageously avoidsregurgitation, i.e. the blood pump does not allow a backflow of blood.For example, in case of left ventricle assistance, during diastole, theblood pump does not allow a black-flow of blood from the aorta back intothe left ventricle.

By applying the zero-flow control mode, the drive unit of the bloodpump, e.g. a rotor or impeller is still spinning. Thus, there is areduced risk of thrombus formation due to the still moving parts.

By means of the zero-flow operation mode, the assistance provided by theblood pump to the heart is set to be basically zero. “Basically zero”means any blood flow still produced has to be at least negligible, butis in any case not negative, i.e. the blood pump does not allowback-flow through the blood pump.

In the zero-flow operation mode, the complete work in overcoming thepressure difference between the pressure in the assisted ventricle, e.g.the left ventricle, and the pressure in the adjacent blood vessel, e.g.the aorta, is provided solely by the heart. This way, the zero-flowoperation mode allows monitoring one or more suitable characteristicparameters of the heart which may be used or interpreted as indicatorfor the status of the heart recovery.

Preferably, the blood flow of the blood pump is related to the drivespeed of the drive unit, e.g. a motor, an electrical current supplied tothe drive unit and/or the pressure difference between the outlet and theinlet of the blood pump. This relation can be stored in a memory, forexample in a look-up table as will be described in more detail below.That is to say, command signal values can be stored in a memory of thecontrol device or a memory in the blood pump accessible by the controldevice.

A first aspect provides a control device for controlling a blood flowQ_(pump)(t) of an intravascular blood pump for percutaneous insertioninto a patient's blood vessel. The blood pump comprises a pump unit anda drive unit for driving the pump unit that is configured to conveyblood from a blood flow inlet towards a blood flow outlet. The controldevice is configured to operate the blood pump in a selectable zero-flowcontrol mode, wherein a blood flow command signal Q_(pump) ^(set)(t) isselected. The control device comprises a first controller and a secondcontroller, wherein the first controller is configured to control theblood flow Q_(pump)(t) by adjusting a speed command signal n_(pump)^(set)(t) for the drive unit, and the second controller is configured tocontrol a drive speed n_(pump)(t) of the drive unit. More specifically,the control device is particularly configured for controlling anintravascular blood pump, or more generally a low inertia device as willbe described in more detail below.

Preferably, the intravascular blood pump comprises, between the bloodflow inlet and the blood flow outlet, a cannula through which the bloodflow is created by the pump unit. In operation, the cannula may extendfor instance through the aortic valve, while the blood flow inlet isdisposed in the left ventricle and the blood flow outlet is disposed inthe aorta.

For example, the controlled blood flow can be constant. By compensatingthe current pressure difference between the blood flow inlet and theblood flow outlet the actual blood flow through the blood pump resultsin a zero-flow. That is to say, in the zero-flow control mode, a currentpressure difference between the blood flow outlet and the blood flowinlet is counteracted by controlling the blood flow via control of thedrive speed.

Preferably, the first controller is configured to determine the speedcommand signal n_(pump) ^(set)(t) based on a difference ΔQ between theblood flow command signal Q_(pump) ^(set)(t) and the blood flowQ_(pump)(t). In other words, the first controller is configured tocompare the actual blood flow Q_(pump)(t) with the blood flow commandsignal Q_(pump) ^(set)(t) to determine the speed command signal n_(pump)^(set)(t).

Preferably, the second controller is configured to control the drivespeed n_(pump)(t) by adjusting a drive current I_(pump)(t) supplied tothe drive unit. For example, the drive unit can comprise a motor, inparticular an electric motor, and the adjusted drive current can be amotor current supplied to the motor. Thus, in case of a rotating driveunit the command speed signal n_(pump) ^(set)(t) and set drive speedn_(pump) ^(set)(t) of the drive unit can be a rotational speed. Themotor may be located in the pump unit and directly or indirectly coupledto the impeller, e.g. by means of a mechanical connection or a magneticcoupling.

Preferably, the first controller and the second controller are part of acascade control system, in which the first controller is an outercontroller and the second controller is an inner controller. The outercontroller may be embedded in outer control loop and may regulate theblood flow generated by the blood pump by comparing the blood flowcommand signal with the generated blood flow and by setting theset-point of an inner control loop, namely the speed command signal ofthe blood pump. The inner controller is part of the inner control loopand may control the speed of the blood pump by adjusting the motorcurrent accordingly.

Preferably, the control device is configured to control the blood flowQ_(pump)(t) for a predetermined zero-flow control period.

For example, the predetermined zero-flow control period may be set tolast a fraction of one cardiac cycle of an assisted heart. That is, thezero-flow control mode is only briefly applied “within-a-beat”. In thiscase, the predetermined zero-flow period is preferably small incomparison with the duration of the heart cycle. This way, informationon the recovery status of the heart may be gathered without any overloadof the heart since the duration without assistance to the heart is keptat a minimum.

For example, the predetermined zero-flow control period may be set tolast at least one complete cardiac cycle or a predetermined number ofcomplete consecutive heart cycles.

Preferably, the control device is configured to synchronize thezero-flow control period with an occurrence of at least onecharacteristic heart cycle event. For example, a beginning and/or end ofthe zero-flow control period is synchronized with the occurrence of theat least one characteristic heart cycle event. Particularly, thebeginning and the end of the zero-flow control period may besynchronized with the occurrence of two characteristic heart cycleevents. This way, the zero-flow control mode can be set to a timeinterval of the cardiac cycle in which a particular characteristicparameter of the heart may provide particular useful information whichdirectly or indirectly indicates the status of heart recovery.

For example, the characteristic heart cycle events may be the opening ofthe aortic valve or the closing of the aortic valve. For example, thecontrol device can be configured to detect the opening of the aorticvalve by one of: presence of equilibrium of the left ventricularpressure and the aortic pressure, the occurrence of the R-wave in anelectrocardiogram, ECG, signal led from the patient with the assistedheart.

Further characteristic heart cycle events may be the opening of themitral valve, the closing of the mitral valve or the occurrence of anend diastolic left ventricular pressure.

Preferably, the control device is configured to monitor values of one ormore characteristic heart parameters. That is, the control device can beconfigured to, in the zero-flow control mode, monitor one or morecharacteristic heart parameters, each time the zero-flow control mode isapplied.

Preferably, the control device is configured to operate theintravascular blood pump in the zero-flow control mode periodically orrandomly. The periodical or random application of the zero-flow controlmode may be performed over a predetermined time span, e.g. fromfractions of a heart cycle up to several days.

Preferably, the control device is configured to identify a trend of theone or more values of monitored characteristic heart parameters. Thetrend of the thereby monitored one or more characteristic heartparameters may be used as an indicator for the recovery status of theheart or the status of heart recovery as such, i.e. one may assistwhether there is a progress in recovery at all. The trend may beindicated to a physician via a user interface of the control device sothat the physician is enabled to decide on the heart recovery status.

For example, the at least one characteristic parameter of the heart canbe the arterial blood pressure measured each time the zero-flowoperation mode is established. By applying the zero-flow control modethe arterial blood pressure may drop. The pressure drop reaching acritical value or showing a critical decrease indicates that it is notpossible to explant the blood pump as the heart hasn't recovered. Inanother example, the arterial blood pressure might stay stable or onlyshows a minor pressure drop during zero-flow control mode. In this caseit can be assumed that the heart has sufficiently recovered, and theblood pump can be explanted.

Preferably, the at least one characteristic heart parameter is at leastone of: the arterial pressure pulsatility AOP|_(max)−AOP|_(min), themean arterial pressure, the contractility of the heartdLVP(t)/dt|_(max), the relaxation of the heart dLVP(t)/dt|_(min), theheart rate HR.

The control device may be configured to measure the blood flowQ_(pump)(t) by means of a sensor, to calculate or to estimate the bloodflow Q_(pump)(t). For example, the pressure difference between the bloodflow outlet and the blood flow inlet may be determined by respectivepressure sensors located at the inlet and the outlet of the blood pump,i.e. a pressure sensor capturing the after-load of the blood pump and apressure sensor capturing the pre-load of the blood pump. The blood pumpmay comprise, alternatively or additionally, one sensor configured tomeasure just the pressure difference directly. Further in thealternative the pressure difference between the blood flow outlet andthe blood flow inlet may be estimated, measured or calculated.

Rather than measuring the blood flow Q_(pump)(t), the blood flowQ_(pump)(t) may be determined using a look-up table, which may representthe relation between the blood flow, the drive speed, and at least oneof a pressure difference between the blood flow outlet and the bloodflow inlet and a drive current supplied to the drive unit. Such lookup-table may include a set of characteristic curves which describe therespective relations, e.g. a set of curves, each for a certain pumpspeed. It will be appreciated that other suitable look-up tables may beused, and the values in the look-up tables may be given in variousunits.

Data for use in a look-up table, such as motor current and blood flow,can be recorded in a test bench assembly by running a blood pump in afluid at a given motor speed and at a defined pump load (pressuredifference between the inlet and the outlet) while recording the flowproduced by the pump. The pump load can be increased over time, e.g.from zero load (no pressure difference between blood flow inlet andblood flow outlet, i.e. maximum flow) to maximum load (no pump function,i.e. no flow), while the motor current and blood flow are recorded. Sucha look-up table may be created for several different motor speeds. Usingsuch look-up table for determining the blood flow Q_(pump)(t) mayprovide an advantageous way of determining the blood flow Q_(pump)(t)during operation of the blood pumps, in particular compared to measuringor calculating the blood flow Q_(pump)(t). With the look-up table, theblood flow Q_(pump)(t) is determined based on easily available operationparameters of the blood pump only. Thus, no sensors for detectingparameters of the patient are necessary, such as pressure sensors fordetecting a pressure difference inside the patient's vessel or flowsensors. Furthermore, reading a value for the blood flow Q_(pump)(t) outof the look-up table does not require computationally intensivecalculation.

However, monitoring the one or more suitable characteristic heartparameter just by applying the zero-flow control mode within-a-beat, theheart may not sufficiently adapt to the missing assistance of the bloodpump. Thus, the monitored characteristic heart parameters may still notsufficiently indicate the true status of heart recovery, e.g. the actualpumping capacity of the heart. Thus, it is possible to repeat thezero-flow control mode within-a-beat in several consecutive heartcycles.

Thus, the predetermined zero-flow period can be set to last at least onecomplete cardiac cycle or a predetermined number of complete consecutiveheart cycles. For example, the predetermined zero-flow period may be setto be a fraction of a heart cycle up to several hours. This way, theheart can fully adapt to the condition of zero assistance by the bloodpump so that the actual status of heart recovery can be betterascertained.

It is also possible to combine zero-flow within-a-beat and over acomplete heart cycle. For example, the zero-flow control mode may beapplied at first for a relatively short time period, for example in afraction of a heart cycle in 1 to 300 consecutive heart cycles. Afteracknowledgement of a natural heart function and a sufficient recoverystatus, the zero-flow control modes can be applied over a longer timeperiod, such as over complete heart cycles for several minutes or hoursup to days.

A second aspect provides a system comprising an intravascular blood pumpfor assistance of a heart and a control device according to the firstaspect.

Preferably, the blood pump is catheter-based i.e. the blood pumppreferably comprises a catheter and a pump unit, preferably with thepump unit located at a distal end of the catheter.

Preferably, the blood pump may be implemented as a rotational bloodpump, i.e. a blood pump driven by a rotational motor.

The blood pump may be catheter-based to be implanted or placed directlypercutaneously into a heart through corresponding blood vessels. Forexample, the blood pump may be a blood pump as published e.g. in U.S.Pat. No. 5,911,685, which is particularly arranged for a temporaryplacement or implantation into the left or right heart of a patient. Asmentioned above, the present invention is particularly useful forintravascular blood pumps, and less relevant for larger VADs that arenot placed inside a blood vessel or inside the heart, but outside thepatient's heart, e.g. implanted in the thoracic cavity.

Preferably, the blood pump is a low inertia device. (a) The blood pumpis a low inertia device by comprising one or more of the followingcharacteristics; (b) Moving, in particular rotating, parts, for examplea rotor or impeller, of the blood pump have low masses by being made ofa low weight material, for example plastic; (c) The drive unit, such asan electric motor, is arranged near, preferably very near, mostpreferably adjacent, to a moving part, for example a rotor or impeller,of the pump unit driven by the drive unit; (d) If the blood pump iscatheter-based, there is no rotational drive cable or drive wire. (e) Acoupling or connection, for example a shaft, of the drive unit with arotating part, for example the rotor or the impeller, of the pump unitdriven by the drive unit is short; and (f) all moving, in particularrotating, parts of the blood pump have small diameters.

A low inertia device particularly includes an intravascular blood pumpfor percutaneous insertion into a patient's blood vessel. Because oftheir small diameter—in particular compared to relatively bulky VADs—allmoving parts of intravascular blood pumps are low weight and are locatednear the rotational axis. This allows to control the pump speed in avery accurate manner since the rotation of the impeller is onlymarginally affected by the inertia of the impeller. That means, onlyslight delay occurs between the command signals and the actual responseof the blood pump. In contrast to that, VADs designed for instance ascentrifugal blood pumps may be bulky and may have a large diameter and,thus, a large rotor with higher mass and may not be referred to as “lowinertia devices”.

One characteristic for a low inertia device is, for instance, thatreducing the pump speed of a low inertia device, in particular quicklyreducing the pump speed, does not require a negative speed signal (whichis typically necessary in large VADs) or other brake command, butreducing the motor current directly results in a reduced pump speed andthe blood pump may be put into the zero-flow control mode simply byreducing the motor current. This is particularly relevant for thewithin-a-beat control as the cardiac cycle is very short and requires ashort response time of the moving parts of the blood pump. Vice versa,it is likewise desirable to quickly accelerate the moving parts, i.e. toquickly increase the pump speed, in order to terminate the zero-flowcontrol mode.

For instance, with a low inertia device in the sense of the presentinvention, it is possible to significantly increase or decrease the pumpspeed within a very short period of time, e.g. within about 50 ms towithin about 100 ms, preferably 60 ms to 80 ms. In other words, the pumpspeed may quickly change or may make a “step change”.

For instance, within said period of time the pump speed may be reducedfrom about 35,000 rpm to about 10,000 rpm, or in another blood pump fromabout 51,000 rpm to about 25,000 rpm, or vice versa accordinglyincreased. However, the duration of the pump speed change is alsodependent on other factors like blood flow, pressure difference,intended amount of pump speed change (i.e. the difference between thepump speed before and after the intended speed change), or the point oftime within the cardiac cycle (as the blood flow is accelerated duringsystole and slowed down during diastole).

A third aspect provides a method for controlling a blood flowQ_(pump)(t) of an intravascular blood pump as discussed with the firstaspect. That is, the blood pump comprises the pump unit with a driveunit and is configured to convey blood from a blood flow inlet towards ablood flow outlet. The method comprises the steps: (i) comparing a setblood flow value Q_(pump) ^(set)(t) with a blood flow value Q_(pump)(t)resulting in a control error e(t) in a first closed-loop cycle; (ii)determining a set speed value n_(pump) ^(set)(t) for the drive meansfrom the control error e(t); controlling a drive speed n_(pump)(t) ofthe drive unit by comparing the set speed value n_(pump) ^(set)(t) withthe drive speed n_(pump)(t) in a second closed-loop cycle.

Preferably, the method further comprises the steps of providing azero-flow mode in which the set blood flow value Q_(pump) ^(set)(t) iszero for a predetermined zero-flow control period, and preferablysetting the predetermined zero-flow control period to last a fraction ofone cardiac cycle of an assisted heart, or to last at least one completecardiac cycle or a predetermined number of consecutive cardiac cyclefractions and/or complete cardiac cycles.

As described above in more detail with respect to the control device,the method may include the step of determining or estimating the bloodflow Q_(pump)(t) using an appropriate look-up table.

Preferably, the first closed-loop cycle is an outer control loop and thesecond closed-loop cycle is an inner control loop of a cascade control.The cascade control system including an outer control loop and an innercontrol loop has been described in more detail above with respect to thecontrol device and are valid also for the method.

Preferably, the method further comprises: synchronizing the zero-flowcontrol period with at least one particular characteristic heart cycleevent.

Preferably, a beginning and/or an end of the zero-flow control period issynchronized with the occurrence of the at least one characteristicheart cycle event.

Preferably, the method further comprises: monitoring one or more valuesof characteristic heart parameters.

Preferably, the method further comprises: identifying a trend in the oneor more monitored values of the characteristic heart parameters.

A fourth aspect provides the control device according to the firstaspect which is configured to carry out the method according to thethird aspect.

The above-discussed functions or functionalities of the control deviceand correspondingly of the control method can be implemented by acorresponding computing unit, in hardware or software or any combinationthereof, of the control device. Such computing unit can be configured bymeans of corresponding computer programs with software code for causingthe computing unit to perform the respectively required control steps.Such a programmable computing unit is basically well known in the artand to the person skilled in the art. Therefore, there is no need todescribe such a programmable computing unit here in detail. Moreover,the computing unit may comprise particular dedicated hardware useful forparticular functions, such as one or more signal processors forprocessing and/or analyzing e.g. the discussed measuring signals.Further, respective units for controlling the speed of a drive of theblood pump may be implemented by respective software modules as well.

The corresponding computer programs can be stored on a data carriercontaining the computer program. Alternatively, the computer program maybe transferred, e.g. via the Internet, in the form of a data streamcomprising the computer program with-out the need of a data carrier.

BRIEF DESCRIPTION OF THE DRAWINGS

Hereinafter the invention will be explained by way of examples withreference to the accompanying drawings.

FIG. 1 shows a block diagram of a feedback control.

FIG. 2 shows an exemplary blood pump laid through the aorta andextending through the aortic valve into the left ventricle together witha block diagram of a control device for the pump speed of the bloodpump.

FIG. 3 shows the exemplary blood pump of FIG. 1 in more detail.

FIG. 4 is an exemplary diagram showing a set of characteristic curvesrepresenting the relationship between the actual pressure differencebetween the in-take and the outlet of the blood pump, the actual pumpspeed of the blood pump, and the blood flow produced through the bloodpump.

DETAILED DESCRIPTION

FIG. 1 shows a block diagram which is an example of the feedback controlloop for blood flow control realized as a cascade control system. Thecontrol loop comprises an outer controller 401 and an inner controller402. The outer controller 401 is embedded in outer control loop andregulates a generated blood flow Q_(pump)(t) of e.g. the blood pump 50shown in FIG. 3 by comparing a blood flow command signal Q_(pump)^(set)(t) with the generated blood flow Q_(pump)(t), and by setting theset-point of the inner control loop, namely the speed command signaln_(pump) ^(set)(t) of the blood pump 50. The inner controller 402 ispart of the inner control loop and controls the speed n_(pump)(t) of theblood pump 50 by adjusting a motor current I_(pump)(t) accordingly.

In the feedback loop shown in FIG. 1, the generated blood flowQ_(pump)(t) is exemplarily calculated by means of a look-up table whichrepresents e.g. the relation of the electrical current I_(pump)(t), thespeed n_(pump)(t) and the generated blood flow Q_(pump)(t).Alternatively or additionally, another look-up table may be used torepresent the relation of the pressure difference between the blood pumpoutlet 56 and the blood pump inlet 54 (cf. FIG. 3), the speedn_(pump)(t) and the blood flow Q_(pump)(t). Another alternative oradditional option for data acquisition of the generated blood flowQ_(pump)(t) is to use a flow sensor.

The flow control regulates the blood flow Q_(pump)(t) through the bloodpump 50 according to the blood flow command signal Q_(pump) ^(set)(t),which can be a constant value (also called set-value) or a changingsignal over time. A constant blood flow set-value Q_(pump) ^(set)(t) maybe in the range of [−5 . . . 10] L/min, preferably in the range of [0 .. . 5] L/min, and most preferably 0 L/min or a very low blood flow aszero-flow.

One of the aims of the here disclosed flow control is to monitor valuesof characteristic parameters of the heart with the implanted pump 50 fordetermining the recovery state of the heart while reducing the effect ofthe pump on the heart function. For this aim, the flow control may use aset blood flow Q_(pump) ^(set)(t) of 0 L/min or a very low blood flow aszero-flow.

It was found, that the inner control loop may have a small time constantrelative to the outer control loop. This way, the inner control loopresponds much faster than the outer control loop. In addition, the innercontrol loop may be performed at a higher sampling rate than the outercontrol loop.

For example, a sampling rate fs_(IN) of the date in the inner controlloop may be in the range of [250 . . . 10 k] Hz, preferably [1 . . . 3]kHz, and most preferably 2.5 kHz.

For example, a sampling rate fs_(OUT) of the data in the outer controlloop may be in the range of [25 . . . 1000] Hz, preferably [100 . . .300] Hz, most preferably 250 Hz.

FIGS. 2 and 3 show an example for a blood pump. The blood pump is anintravascular blood pump configured for a percutaneous insertion into aheart. In the embodiment shown, the blood pump is a micro axial rotaryblood pump, in the following for short called blood pump 50. Such ablood pump is, for example, known from U.S. Pat. No. 5,911,685 A.

The blood pump 50 is based on a catheter 20 by means of which the bloodpump 50 can be temporarily introduced via a vessel into a ventricle of apatient's heart. The blood pump 50 comprises in addition to the catheter20 a rotary drive unit 51 fastened to the catheter 20. The rotary driveunit 51 is coupled with a pump unit 52 located at an axial distancetherefrom.

A flow cannula 53 is connected to the pump unit 52 at its one end,extends from the pump unit 52 and has blood flow inlet 54 located at itsother end. The blood flow inlet 54 has attached thereto a soft andflexible tip 55.

The pump unit 52 comprises the pump housing with blood flow outlet 56.Further, the pump unit 52 comprises a drive shaft 57 protruding from thedrive unit 51 into the pump housing of the pump unit 52. The drive shaft57 drives an impeller 58 as a thrust element. During operation of theblood pump 50 blood is sucked through the blood flow inlet 54, conveyedthrough the cannula 53 and discharged through the blood flow outlet 56.The blood flow is generated by means of the rotating impeller 58 drivenby the drive unit 51.

In the embodiment shown, through the catheter 20 pass three lines,namely two signal lines 28A, 28B and a power supply line 29 for supplingelectrical power to the drive unit 51 of the blood pump 50. The signallines 28A, 28B and the power-supply line 29 are attached at theirproximal end to the control device 100 (FIG. 2). The signal lines 28A,28B are associated with respective blood pressure sensors withcorresponding sensor heads 30 and 60, respectively. The power supplyline 29 comprises supply lines for supplying electrical power to thedrive unit 51.

The drive unit 51 may be a synchronous motor. In an exemplaryconfiguration the electrical motor may comprise several motor windingunits for driving the impeller 58 that is coupled with the drive shaft57. A rotor of the synchronous motor may comprise at least one fieldwinding or, alternatively, a permanent magnet in case of a permanentmagnet excited synchronous motor.

In a preferred embodiment, the blood pump 50 is a catheter-based microaxial rotational blood pump for percutaneous insertion through apatient's vessel into the patient's heart. Here, “micro” indicates thatthe size is small enough so that the blood pump can be percutaneouslyinserted into the heart, e.g. into one of the ventricles of the heart,via blood vessels leading to the heart. This also defines the blood pump50 as an “intravascular” blood pump for percutaneous insertion. Here,“axial” indicates that the pump unit 52 and the drive unit 51 driving itare arranged in an axial configuration. Here, “rotational” means thatthe pump's functionality is based on a rotating operation of the trustelement, i.e. the impeller 58, driven by the rotational electrical motorof the drive unit 51.

As discussed above, the blood pump 50 is based on the catheter 20 bywhich the insertion of the blood pump 50 through the vessels can beperformed and through which the power supply line 29 can be passed forsupplying electrical power to the drive unit 51 and control signals,e.g. from the drive unit 51 and the sensor heads 30, 60.

As mentioned above, the present invention is particularly configured forintravascular blood pumps, such as the blood pump 50 shown in FIG. 3,and less configured or even not suitable for VADs, which are implantedoutside the patient's heart, e.g. centrifugal blood pumps connected tothe patient's heart and placed in the thoracic cavity and operate indifferent ranges of pump speed. As explained herein, this isparticularly because of inertia effects which significantly affect thefunction of large VADs but can be avoided in low inertia devices, suchas intravascular blood pumps.

As shown in FIG. 2, each signal line 28A, 28B connects to one respectiveblood pressure sensor with the corresponding sensor head 30 and 60,respectively, which are located externally on the housing of the pumpunit 52. The sensor head 60 of the first pressure sensor is connectedwith signal line 28B and is for measuring the blood pressure at theblood flow outlet 56. The sensor head 30 of the second blood pressuresensor is connected with signal line 28A and is for measuring the bloodpressure at the blood flow inlet 54. Basically, signals captured by thepressure sensors, which carry the respective information on the pressureat the location of the sensor and which may be of any suitable physicalorigin, e.g. of optical, hydraulic or electrical, etc., origin, aretransmitted via the respective signal lines 28A, 28B to correspondinginputs of a data processing unit 110 of the control device 100. In theexample shown in FIG. 2, the blood pump 50 is positioned in the aortaand via the aortic valve in the left ventricle of the heart so that thepressure sensors are arranged for measuring the aortic pressure AoP(t)by sensor head 60 and the left ventricular pressure LVP(t) by sensorhead 30.

The data processing unit 110 is configured for acquisition of externaland internal signals, for signal processing, which includes for examplecalculation of a difference between pressure signals as a basis forestimating the generated blood flow Q_(pump)(t) which may serve ascontrol signal for the flow control approach, for signal analysis todetect the occurrence of characteristic events during the cardiac cyclebased on the acquired and calculated signals, and for generating triggersignals σ(t) for triggering a speed command signal generator 120, justto name a few examples.

For the given example of a flow control approach, the speed commandsignal generator 120 represents the outer controller 401 in FIG. 1.

In the shown embodiment, the data processing unit 110 is connected viacorresponding signal lines to additional measurement devices which aredepicted in general by 300. Such additional measurement devices are, inthe embodiment, a patient monitoring unit 310 and an electrocardiograph(ECG) 320; apparently, these two devices 310 and 320 are just twoexamples and not exhaustive, i.e. other measuring devices may be usedfor providing useful signals, as well. The depicted ECG 320 provides anECG signal ECG(t) to the data processing unit 110.

The control device 100 further comprises a user interface 200. The userinterface 200 for interaction with the user of the device. The userinterface 200 comprises as output means a display 210 and as input meansa communication interface 220. On the display 210, values of settingparameters, values of monitored parameters, such as measured pressuresignals, and other information is displayed. Further, by thecommunication interface 220, the user of the control device 100 isenabled to take control of the control device 100, e.g. by changing thesetup and settings of the whole system comprised of the blood pump andthe control device 100.

For the given example of a flow control approach, a setting would be thechoice of the desired pump flow Q_(pump) ^(set)(t) in FIG. 1.

The data processing unit 110 is particularly configured to derive orpredict the time of occurrence of one or more predefined characteristicevents during the cardiac cycle of the assisted heart. For example, thedata processing unit 110 is configured to detect a predefinedcharacteristic cardiac cycle event during the cardiac cycle by means ofreal-time analysis of monitored signals. Alternatively or additionally,a predefined characteristic cardiac cycle event, such as e.g. theR-wave, may be identified by the ECG signal from the ECG 320.

The occurrence of one or more determined predefined characteristicevents are used for generation of a particular trigger signal σ(t) or asequence of trigger signals σ(t). The resulting trigger signal σ(t) (orsequence thereof) is forwarded to the speed command signal generator 120to correspondingly trigger speed command signal changes provided to aspeed control unit 130.

In the context of the present invention, the speed command signalgenerator 120 is configured to operate the blood pump 50 in thezero-flow control mode.

The data processing unit 110 may be configured to predict the time ofoccurrence of the at least one predefined characteristic cardiac cycleevent in an upcoming cardiac cycle based on the stored information aboutthe characteristic cardiac cycle events occurring during the currentand/or previous cardiac cycles, and analyze previous values of thesespeed command signals n_(pump) ^(set)(t), as well.

For example, a characteristic cardiac cycle event may be the beginningof contraction of the heart at the beginning of the systolic phase. Thedetected occurrence or the predicted occurrence of such characteristiccardiac cycle event can be used for synchronizing sequentialapplications of a particular control approach for the blood pump 50within one or several cardiac cycles or within a particular timeinterval of the cardiac cycle.

Correspondingly, the speed command signal generator 120 is configured toadjust the speed command signal n_(pump) ^(set)(t) for the blood pump 50to control the generated blood flow Q_(pump)(t) according to a givenblood flow command signal Q_(pump) ^(set)(t) which maybe set to be e.g.0 L/min.

To control the generated blood flow Q_(pump)(t), the speed commandsignal generator 120 is configured as an outer controller in the cascadecontrol system to provide a suitable speed command signal n_(pump)^(set)(t) to the speed control unit 130 either in a time-continuous wayby continuously controlling the generated blood flow Q_(pump)(t) (as afirst setup) or in an event-based switching control manner (as a secondsetup).

In the first setup, the command signal generator 120 continuouslyprovides the speed command signal n_(pump) ^(set)(t) to the speedcontrol unit 130 as part of a cascaded blood-flow control system beingfed with external and internal signals by the data processing unit 110.

In the second setup, the speed command signal generator 120 operates asin the first setup with the additional feature to switch the continuousblood flow control on and off.

In the zero-flow control mode, the speed command signal n_(pump)^(set)(t) is continuously adjusted by the flow controller in the outercontrol loop. The on/off switching is triggered by at least one triggersignal σ(t) provided by the data processing unit 110.

The second setup is suitable, if the zero-flow control is just appliedfor a short time interval, in particular short compared with theduration of one cardiac cycle; in other words, the generated blood flowis controlled with a blood flow command signal Q_(pump) ^(set)(t) 0L/min just for a brief time interval within the cardiac cycle(within-a-beat blood flow control).

The speed control unit 130 controls the speed n_(pump)(t) of the bloodpump 50, in accordance with the speed command signal n_(pump) ^(set)(r),by supplying an electrical current I_(pump)(t) to the drive unit 51 ofthe blood pump 50 via the power-supply line 29.

The current level of the supplied motor current I_(pump)(t) correspondsto the electrical current currently required by e.g. an electrical motorof the drive unit 51 to establish a target speed level as defined by thespeed command signal n_(pump) ^(set)(t). A measuring signal such as thesupplied motor current I_(pump)(t) can be used as a representativesignal of an internal signal of the control device 100 and may beprovided to the data processing unit 110 for further processing. Via thepower-supply line 29, the blood pump 50 may also communicate with thecontrol unit 100.

Basically, amongst others, the control device 100 is configured tooperate the blood pump 50 in the selectable zero-flow control mode, inwhich the blood flow Q_(pump)(t) of the blood pump 50 is controlled tocounteract the changing pressure difference between the blood outlet 56and the blood inlet 54 due to heart beat which can be regarded asdisturbance. The blood flow Q_(pump)(t) is controlled by adjusting thespeed command signal n_(pump) ^(set)(t). As proposed herein, the controldevice 100 is configured to control the blood flow Q_(pump)(t) of theblood pump 50 such that the blood pump 50 generates zero blood flow fora predetermined zero-flow control period.

In the first setup with continuous flow control, the predeterminedzero-flow control period is set to last at least one complete cardiaccycle or a predetermined number of complete consecutive heart cycles.Further in the first setup, the control device 100 is configured tomonitor the values of one or more characteristic heart parameters withthe implanted blood pump 50. Again, the monitored values of one or morecharacteristic heart parameters can be used as an indicator for thestatus of heart recovery.

In the second setup with an event-based zero-flow control, thepredetermined zero-flow control period is set to be a fraction of theduration of one cardiac cycle of the heart with the implanted blood pump50. In this setup, the control device 100 is configured to synchronizethe beginning and the end of the zero-flow control period with theoccurrence of a particular characteristic heart cycle event.

Notably, the control device 100 may control the blood flow Q_(pump)(t)through the blood pump 50 periodically or randomly.

In a particular implementation, a characteristic heart cycle event isthe opening of the aortic valve or the closing of the aortic valve orthe opening of the mitral valve or the closing of the mitral valve, orparticular pressure values as the end-diastolic left ventricularpressure.

Further in the second set-up, as in the first setup, the control device100 is configured to monitor the values of one or more characteristicheart parameters of the heart with the implanted blood pump 50 duringthe zero-flow control period. The monitored values of one or morecharacteristic heart parameters can be used as an indicator for thestatus of heart recovery, as well.

The control device 100 is further configured to identify a trend in thevalues of one or more monitored characteristic parameters. As mentionedherein above, the trend can be interpreted as an indicator for thestatus of heart recovery, too.

In any case, for implementing the zero-flow control mode, the controldevice 100 is configured to control the blood flow Q_(pump)(t) byadjusting the speed command signal n_(pump) ^(set)(t) of the blood pump50 whereby the drive speed n_(pump)(t) is affected by the changing bloodpressure difference between the blood flow outlet 56 of the blood pump50 and the blood flow inlet 54 of the blood pump 50 during the cardiaccycle.

Particularly, the control device 100 is configured to determine theblood flow Q_(pump)(t) of the blood pump 50 based on predeterminedsignals as e.g. the drive speed n_(pump)(t), the electrical currentI_(pump)(t) and/or the pressure difference between the blood flow outlet56 and the blood flow inlet 54 of the blood pump 50.

FIG. 4 is an exemplary diagram showing a set of characteristic curvesrepresenting the relationship between the pressure differenceΔP_(pump)(t) between the blood flow outlet 56 and the blood flow inlet54 of the blood pump 50, the drive speed n_(pump)(t) of the blood pump50, and the blood flow Q_(pump)(t) generated by the blood pump 50, e.g.through the flow cannula 53 in FIG. 3.

In order to perform the zero-flow control, the data processing unit 110is configured to continuously determine the blood flow Q_(pump)(t)generated by the blood pump 50, based on the known the speedn_(pump)(t), the known electrical current I_(pump)(t) supplied to thepump unit 51 and/or the monitored pressure difference ΔP_(pump)(t)between the blood flow outlet 56 and the blood flow inlet 54 of theblood pump 50. A set blood flow value Q_(pump) ^(set)(t) could be zeroor at least a positive value close to zero.

For example, based on FIG. 4, in the case, the monitored pressuredifference ΔP_(pump)(t) between the blood flow outlet 56 and the bloodflow inlet 54 of the blood pump 50 is 60 mmHg, the drive speedn_(pump)(t) has to be at about 20.000 1/min (rpm) to generate a bloodflow Q_(pump)(t) of about 0 L/min.

It will be appreciated that the values, relationships and shapes of thecurves shown in the characteristic diagram of FIG. 4 are only exemplaryand may vary depending on the used blood pump, patient or other factors.In particular, each and every blood pump, even blood pumps of the sametype, may have individual characteristic diagrams, i.e. the look-uptable may be pump specific. Furthermore, once implanted in a patient,the characteristic diagram might have to be adapted by means of apatient specific correction factor, including various factors likeviscosity of the blood, location of the blood pump etc. Using acorrection factor may increase the accuracy of the flow estimationobtained from the look-up table.

As discussed before, the current pressure difference ΔP_(pump)(t) can bedetermined by means of pressure sensors (e.g. sensors 30, 60, FIG. 3) ofthe blood pump 50. Thus, speed control unit 130 can be continuouslyprovided with values out of storage unit, e.g. a look-up table in whichthe characteristic curves of FIG. 4 (here representing the relationshipbetween the above-discussed values ΔP_(pump)(t), Q_(pump)(t), andn_(pump)(t)) are stored. The storage unit may be a read only memory ofthe data processing unit 110 or, alternatively, a storage chip in theblood pump 50 or in the control console 130 thereof.

The at least one characteristic heart parameter value is at least oneof: the arterial blood pressure measured each time the zero-flowoperation mode is established.

Preferably, the blood pump 50 is a low inertia device. This particularlyachieved in that moving, in particular rotating, parts, for example arotor or impeller, of the blood pump 50 comprise low masses, by beingmade of a low weight material, for example plastic. Additionally, thedrive unit, such as an electric motor, is arranged near, preferably verynear, most preferably adjacent, to a part, such as the trust element,for example a rotor or impeller 58, driven by the drive unit.Additionally, even though the blood pump 50 is catheter-based, there isno rotational drive cable or drive wire. Additionally, a coupling orconnection, for example the shaft 57, of the drive unit 51 with thetrust element, for example a rotor or impeller 58, driven by the driveunit 51 is kept short. Additionally, all moving, in particular rotating,parts of the blood pump 50 have small diameters.

Summarizing, in the herein proposed zero-flow control approach, thecontrol device 100 controls the generated blood flow Q_(pump)(t) throughthe blood pump 50 in a cascade control consisting of an outer and aninner control loop. That means the generated blood flow Q_(pump)(t)through the blood pump 50 is controlled by adjusting the speed commandsignal n_(pump) ^(set)(t) for the drive unit 51 of the blood pump 50 inthe outer control loop, and the drive speed n_(pump) (t) by adjustingthe electrical current I_(pump)(t) is controlled in the inner controlloop. The zero-flow control approach is either applied continuously orpartly continuously, i.e. the zero-flow control period lasts either oneor several complete cardiac cycles or just a fraction of a cardiaccycle. In the case, that the predetermined zero-flow control periodlasts just a portion of the duration of the cardiac cycle, the zero-flowcontrol period may be synchronized with the heartbeat by means of atleast one characteristic event of the cardiac cycle.

We claim:
 1. A control device for controlling a blood flow of anintravascular blood pump for percutaneous insertion into a patient'sblood vessel, the blood pump comprising a pump unit and a drive unit fordriving the pump unit that is configured to convey blood from a bloodflow inlet towards a blood flow outlet, wherein the control device isconfigured to operate the blood pump in a selectable zero-flow controlmode, wherein a blood flow command signal is selected, and the controldevice comprises a first controller and a second controller, wherein thefirst controller is configured to control the blood flow by adjusting aspeed command signal for the drive unit, and the second controller isconfigured to control a drive speed of the drive unit.
 2. The controldevice of claim 1, wherein the first controller is further configured todetermine the speed command signal based on a difference between theblood flow command signal and the blood flow.
 3. The control deviceclaim 1, wherein the second controller is configured to control thedrive speed by adjusting a drive current supplied to the drive unit. 4.The control device of claim 1, wherein the first controller and thesecond controller are part of a cascade control system, in which thefirst controller is an outer controller and the second controller is aninner controller.
 5. The control device of claim 1, wherein the controldevice is configured to control the blood flow for a predeterminedzero-flow control period.
 6. The control device of claim 5, wherein thepredetermined zero-flow control period is set to last a fraction of onecardiac cycle of an assisted heart; or the predetermined zero-flowcontrol period is set to last at least one complete cardiac cycle or apredetermined number of complete consecutive heart cycles.
 7. Thecontrol device of claim 6, wherein the control device (100) isconfigured to synchronize the zero-flow control period with anoccurrence of at least one characteristic heart cycle event.
 8. Thecontrol device of claim 7, wherein at least one of a beginning or an endof the zero-flow control period is synchronized with the occurrence ofthe at least one characteristic heart cycle event.
 9. The control deviceof claim 8, wherein the at least one characteristic heart cycle event isthe opening of the aortic valve or the closing of the aortic valve. 10.The control device of claim 1, wherein the control device is configuredto monitor values of one or more characteristic heart parameters. 11.The control device of claim 10, wherein the control device is configuredto operate the blood pump in the zero-flow control mode periodically orrandomly.
 12. The control device of claim 11, wherein the control deviceis configured to identify a trend of the one or more values of monitoredcharacteristic heart parameters.
 13. The control device of claim 12,wherein the at least one characteristic heart parameter is at least oneof: the arterial pressure pulsatility, the mean arterial pressure, thecontractility of the heart, the relaxation of the heart, or the heartrate.
 14. The control device of claim 1, wherein control device isconfigured to measure the blood flow by means of a sensor or tocalculate or estimate the blood flow.
 15. The control device of claim 1,wherein control device is configured to determine the blood flow using alook-up table which represents the relation between the blood flow, thedrive speed, and at least one of a pressure difference between the bloodflow outlet and the blood flow inlet or a drive current supplied to thedrive unit.
 16. A system comprising an intravascular blood pump forpercutaneous insertion into a patient's blood vessel and a controldevice for controlling a blood flow of the intravascular blood pump, theblood pump comprising a pump unit and a drive unit for driving the pumpunit that is configured to convey blood from a blood flow inlet towardsa blood flow outlet, wherein the control device is configured to operatethe blood pump in a selectable zero-flow control mode, wherein a bloodflow command signal is selected, and the control device comprises afirst controller and a second controller, wherein the first controlleris configured to control the blood flow by adjusting a speed commandsignal for the drive unit, and the second controller is configured tocontrol a drive speed of the drive unit.
 17. The system of claim 16,wherein the blood pump comprises one or more moving parts and is a lowinertia device having one or more of the following characteristics: theone or more moving parts of the blood pump being made of a low weightmaterial; being arranged near to at least one moving part of the one ormore moving parts of the blood pump without a rotational drive cable;the drive unit being coupled by a short connection to at least onemoving part of the one or more moving parts; and the one or more movingparts of the blood pump having small diameters.
 18. A method forcontrolling a blood flow of an intravascular blood pump for percutaneousinsertion into a patient's blood vessel, the blood pump comprising apump unit with a drive unit and being configured to convey blood from ablood flow inlet towards a blood flow outlet, wherein the methodcomprises the steps: comparing a set blood flow value with a blood flowvalue resulting in a control error in a first closed-loop cycle,determining a set speed value for the drive means from the controlerror, controlling a drive speed of the drive unit by comparing the setspeed value with the drive speed in a second closed-loop cycle.
 19. Themethod of claim 18, further comprising: providing a zero-flow mode inwhich the set blood flow value is zero for a predetermined zero-flowcontrol period.
 20. The method of claim 18, wherein the firstclosed-loop cycle is an outer control loop and the second closed-loopcycle is an inner control loop of a cascade control.
 21. The method ofclaim 19, further comprising: synchronizing the zero-flow control periodwith at least one particular characteristic heart cycle event.
 22. Themethod of claim 21, wherein at least one of a beginning or an end of thezero-flow control period is synchronized with the occurrence of the atleast one characteristic heart cycle event.
 23. The method of claim 18,further comprising: monitoring one or more values of characteristicheart parameters.
 24. The method of claim 23, further comprising:identifying a trend in the one or more monitored values of thecharacteristic heart parameters.
 25. (canceled)
 26. The system of claim17, wherein the one or more moving parts of the blood pump comprises arotor or an impeller.
 27. The method of claim 19, further comprising:setting the predetermined zero-flow control period to last a fraction ofone cardiac cycle of an assisted heart, or to last at least one completecardiac cycle or a predetermined number of at least one of consecutivecardiac cycle fractions or complete cardiac cycles.